This invention concerns the field of wide spectrum radio frequency signal receivers having signal frequency identifying capability and possible electronic warfare usage.
In the field of electronic warfare a broadband radio receiver such as the "Instantaneous Frequency Measurement" (IFM) receiver is often used as a defensive tool to sense the illumination of a target by an incoming radio frequency signal such as a radar signal. This same receiver is also used to analyze the nature of the illuminating signal with respect to its carrier frequency, pulse width and other characteristics. The major deficiency in instantaneous frequency measurement receivers is that the receiver can only read out one input signal at a time.
If multiple signals arrive at the input of an instantaneous frequency measurement receiver, the best result possible is that the receiver reports only one of the input frequencies correctly and the receiver may even report an erroneous frequency that is not related to either one of the input signals--all without the operator being alerted to a difficulty. This and other characteristics of an instantaneous frequency measurement receiver may be better understood from a consideration of my U.S. Pat. No. 5,499,391 which is concerned with a digitized arrangement of an instantaneous frequency measurement receiver; the contents of which are hereby incorporated by reference herein. Various improvements have been suggested to detect and announce the existence of simultaneous signal difficulties in such a receiver, including assigning a single binary bit to values in the input data sequence as disclosed in one of my previous patent filings. However, these previous improvements have achieved only limited degrees of success, being unable to characterize two signals or being so hardware intensive and impractical that, after a period of frustration have, therefore, provided an incentive for the present approach to realization of a hardware efficient broadband electronic warfare receiver.
The role of an improved electronic warfare radio receiver in the present environment is, therefore, to provide an operator with information indicating that a search signal, a signal originating from, for example, a hostile radar apparatus, has found or at least been received at the operator's location. This search signal may be as brief in its time duration as a single pulse of radio frequency carrier signal--as indeed is in accordance with the practice used in modern stealth-considered radar equipment. From this brief received signal, it is nevertheless desirable to determine as much information as possible about the signal's originating radar apparatus, especially such information as the carrier frequency and the pulse characteristics of the radar.
The presence of two signal sources of close time concurrence and different frequency content is readily accommodated in the new type of radio receiver disclosed in the present document. This receiver is based on real time application of the discrete Fourier Transform mathematical function with a single binary bit kernel function. The Fourier transform concepts used in the present invention receiver are novel in that the mathematical operations performed to accomplish the Fourier transform are addition and subtraction, the requirement for multiplication operations in performing the Fourier transform has been eliminated. Not only is the receiver of the present invention able to process two signals simultaneously, it is simple and has the potential to be fabricated on a single chip.
The receiver of the present invention is perceived to fulfill a need in this field for a more reliable and foolproof apparatus for detection of and characteristic identification of incoming signals, an apparatus which performs in a reliable and predictable manner in the presence of one or two incoming radio frequency signals and is suitable for in the field usage. The present invention departs from the phase operated concepts of the IFM receiver and is believed to provide greater reliability and other advantages while also providing a receiver of relatively small physical size and overall simplicity.
The U.S. patent art indicates the presence of significant inventive activity in the area of broad spectrum radio receivers and in radio receivers and other apparatus which employ a Fourier transform function in order to examine frequency-related aspects of incoming signals. The present invention is, however, believed distinguished over the several inventions of this prior art by its specific treatment of the Fourier transform function and by the large demands placed on this Fourier transform realization in the real-time environment of a microwave band electronic warfare receiver.